CA2861450C - Mixed salt co2 sorbent, processes for making and uses thereof - Google Patents

Abstract

A mixed salt composition which is useful as a CO2 sorbent is described. The mixed salt composition comprises a Mg salt, and at least one Group IA element salt, where the Mg and Group IA element are present at a molar ratio of from 3:1 to 8:1. The resulting composition can adsorb about 20% or more of C02 in a gas. Via varying the molar ratios of the components, and the Group IA element, one can develop compositions which show optional functionality at different conditions. The composition is especially useful in the adsorptive capture of CO2 on mobile sources, such as transportation vehicles, where it can be recovered during regeneration of the adsorbent composition and the CO2 used as a coolant gas, as a reactant in manufacture of fuel, and so forth.

Description

MIXED SALT CO2 SORBENT, PROCESSES FOR
MAKING AND USES THEREOF
RELATED APPLICATIONS
[01] [BLANK]
FIELD OF THE INVENTION
[021 This invention relates to materials which are useful in removing CO2 from gas. More particularly, it relates to mixed salt compositions which act as sorbents for the CO2, methods for making these materials and their uses. These adsorbents are useful for removing CO2 from exhaust gas stream in both stationary and mobile applications, such as transportation vehicles, and so forth.
BACKGROUND AND PRIOR ART
[03] Reduction of CO2 emissions is a key goal for all scientific disciplines, acerbated by the growing evidence of, and concern over, climate change induced by CO2.
It is estimated that, in the United States, about 1/3 of CO2 emissions are generated by the transportation sector through combustion of fuels.
[04] One approach to mitigating these emissions is to capture as much CO2 as possible from exhaust gases, prior to their release to the atmosphere. Post-combustion, CO2 capture technologies are being developed for application to stationary sources of CO2. These sources include coal, and natural gas fired power plants, as well as processes for production of materials as diverse as cement and steel. This invention as described herein is useful in such applications, but also in mobile source applications.
[05] Primary challenges for developing useful processes for capture of CO2 from stationary sources, include energy demand, and capital expenditures.
Developing technologies which address CO2 capture from mobile sources involves these factors, as well as space limitation, the dynamics of operating conditions, parameters such as high temperature and low pressures, and so forth_ High temperatures and low pressures are especially important considerations, because these are outside of the operating and optimum ranges of most CO2 capture technologies.
1061 It is a purpose of the invention described herein to provide an effective, low cost CO2 "scrubbing" technology which addresses these issues. How the invention achieves this will be seen in the disclosure which follows.
SUMMARY OF THE INVENTION
107] The invention relates to a mixed salt, solid sorbent composition which is useful in removing CO2 from gases. The composition may be regenerated easily, and thus is useful in continuous scrubbing processes. Further, it is useful in removal of CO2 from both mobile and stationary applications. An important feature of the invention is that it is useful in the removal of CO2 from gas mixtures at ambient or near ambient pressures, and at temperatures ranging from about 50 C to about 400 C, with release of the CO2 at temperatures of from about 150 C to about 500 C.
1081 The mixed salt sorbent compositions of the invention contain alkaline earth and alkali metals, in salt form and at a range of ratios relative to each other.
Preferably, the alkaline earth metal is represented by magnesium (Mg), while the alkali metal is one of the "group IA" elements, i.e., Li, Na, K, or Rb.
1091 As used herein, the term "stationary applications" includes coal, oil, and gas-fired power generating plants, steam boilers for commercial and industrial use, heat plants and other such installations that produce CO2 from the combustion of hydrocarbon fuels.
[10i The term "mobile sources" includes internal combustion engines used to power all types of vehicles such as automobiles, trucks, buses, trains, boats, and airplanes that produce an exhaust gas stream containing CO2 from the combustion of hydrocarbon fuels.

2 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1111 This example details the preparation and use of a CO2 sorbent according to the invention.
1121 An amount (395g) of magnesium carbonate hydroxide (MgCO3 =
Mg(OH)2 x H20) was added to 800 ml of a solution of sodium carbonate (42.18 g) and sodium nitrate (21.63 g) dissolved in deionized water. This produced a mixed salt slurry which was stirred for 30 minutes. The slurry was then covered and allowed to sit for 16 hours at ambient temperature, after which it was dried, at 120 C, for 16 hours, to form a dry cake of MgO:Na2CO3:NaNO3. Analysis showed a mass ratio of 75.8:16:8.2 and a molar ratio of Mg:Na of about 4.8. This dry cake was then calcined by heating from 120 C to 450 C, at a ramp rate of 3 C/minute, followed by 450 C for 4 hours.
The calcined cake was crushed and sieved to collect a 150-425 mesh fraction, which was then tested.
[13] The testing involved loading a packed bed reactor with 6g of the sorbent described supra, with inert SiC added to occupy any remaining volume. A
conventional gas analyzer was used to measure the concentration of CO2 leaving the reactor.
The reactor was then activated by heating it to 450 C, at a rate of 10 C/minute, using a flow of N2 and was held at this temperature until the concentration of CO2 in the effluent dropped below 0.1%. When the CO2 concentration dropped below 0.1%, the reactor was cooled to the lowest adsorption temperature tested and then a simulated exhaust gas (13%
CO2, 13% H2O, remainder N2), was added to the reactor as a feedstream. The concentration of CO2 in the gas effluent was measured continuously and the adsorption phase was continued, until the concentration of CO2 in the effluent was 90% of the concentration in the feed gas, i.e., the "90% break through." When this point was reached, the feed gas was changed to pure N2, and its temperature was ramped at C/minute to 450 C. The reactor itself was maintained at 500 C and until the effluent gas had a CO2 concentration below 0.1% vol., or for 2 hours, so as to regenerate the

3 sorbent. The reactor temperature was then decreased to the desired adsorption temperature, and the process was repeated.
[14] Figure 1 shows the amount of CO2 loaded on the sorbent, over a temperature range of 200-425 C, at 25 C increments.

[151 The prior example described the preparation of the CO2 sorbent via gelation. This example describes a preparation method using precipitation, to form the same mixed salt composition.
[16] A solution of 233.4 g of Na2CO3 in 3000 ml deionized water was placed in a 5.0 liter plastic beaker, and stirred vigorously with a mechanical agitator.
A second solution, of 188.4 g Mg(NO3)2 : 6 H20 in 500 ml of deionized water, was pumped into the first solution, at a rate of approximately 30 ml/minute. A slurry resulted which was stirred for an hour. The slurry was stored, overnight, as described supra, and then filtered to yield a wet precipitate cake. About 3200 mls of filtrate were collected.
This was dried, at 120 C for 24 hours to form a dry cake, which was treated a described in the first example, supra. The sorbent was then tested, as in Example 1. Figure 2 shows these results.
[17] In additional tests, the effect of the alkali element in the mixed salt sorbent composition was evaluated by using Li, Na, or K salts to prepare the final products. Salts were prepared in the manner set forth, supra, using a molar ratio of Mg:
alkali metal of 6:1. The resulting products were tested for their ability to remove CO2 from the simulated exhaust gas described supra. Adsorption was carried out at temperatures ranging from 100 - 450 C, at GHSV of 3,125/hour. The sorbents were regenerated by ramping temperatures to 450 C, at a rate of 10 C/minute, at a GHSV of 2500/hour.
[18] Figure 3 presents these results. While Na presents the best range of operation, the other alkali metals tested function well at different temperatures.

4

5 [19] In yet further experiments, the effect of the source of magnesium on the resulting sorbent was tested. The mixed salt compositions of the invention should have either Mg2CO3 or MgO as a component thereof. Hence, in the reaction producing these sorbents, one selects a Mg compound is selected which will, preferentially, lead to one of these.
[20] Mg(NO3)2, MgO, and Mg(OH)2 were all tested, using the same parameters of Examples 1 and 2.
[21] Figure 4, which presents these results, shows that the nitrate salt produced a sorbent with a significantly greater ability to adsorb CO2.
[22] It is noteworthy that Mg(NO3)2 has significantly greater solubility in water than the other compounds. The differences in solubility also indicate that the final products result from different reactive mechanisms. The nitrate salt, for example, participates in anion exchange with the sodium salts, whereas the oxide and hydroxide do not. Hence, the more soluble the magnesium salt, the greater the adsorption ability of the final product. Mg(NO3)2, MgC12, Mg(CH3C00)2, and other highly soluble magnesium salts are thus preferred in making the sorbents of the invention.
[23] Further experiments were carried out to understand the role of Na in the adsorbence process. These studies were motivated by the recognition that, when CO2 is adsorbed onto the compositions, it is loaded in the form of MgCO3, while the sodium species do not store the CO2.
[24] Sorbents were prepared, as described, supra, using Mg:Na molar ratios of 3:1 to 8:1, and were tested as described in these examples.
[25] It was found that when the Mg:Na ratio was from 8:1 - 6:1, the performance followed that of the examples, i.e., CO2 loading capacity increased with adsorption temperature, reaching a maximum of about 13 wt%, at 350 C, followed by a rapid decrease in capacity as temperatures increased.

[26] Decreasing the Mg:Na ratio to 4:1 - 3:1 impacted loading capacity significantly; however, a decrease in Na content also resulted in a shift to maximum adsorbency at 250 C, as shown in Figure 5. These findings suggest that different mechanisms are involved.
[27] An additional process parameter which was investigated in the study of the invention was the concentration of the reactants in the precipitating solution.
1281 Comparative tests were undertaken where, as above, the molar ratio of Mg:Na remained 6:1, but where the concentration of the reactants in the solution was 0.05, 0.1, 0.2, and 0.3M. Reactions proceeded as per Example 2, supra, to precipitate mixed salt sorbent materials. The resulting materials were then tested as in Examples 1 and 2.
[29] The results, set forth in Figure 6, shows that concentration had a profound impact on the performance of the sorbents. All functioned, but had optional activity at different temperatures, with a decrease in concentration reducing peak CO2 loading temperature to from 250 C to 275 C, as well as an increase in CO2 loaded on the sorbent, from about 12-13 wt% to about 20 wt%.
[30] While the experiments, supra, used Na2CO3 as a precipitating agent, others can be used, as was exemplified with (NH4)2CO3.
[31] To prepare the sorbents with Na2CO3, the precipitating agent was added slowly in the form of a solution, to a solution of MgNO3. For (NH4)2CO3, this was added to a solution of MgNO3 and Na2NO3. The Mg:Na molar ratio of 6:1 was maintained.
[32] The results, depicted in Figure 7 showed that the product obtained with Na2CO3 exhibited a broad range of activity, whereas that prepared with (NH4)2CO3 showed a very sharp spike in activity at 300 C, and very little activity at other temperatures. These results suggest that changes in the precipitating agent can be used to prepare sorbents for different applications.

6 [33] The foregoing disclosure sets forth various features and embodiments of the invention, including a mixed salt composition useful as a sorbent for gaseous CO2.
The composition of the invention comprises a mixed salt of a magnesium compound, such as MgCO3 or MgO, and at least one salt of a Group TA metal, wherein the molar ratio of Mg to the Group IA metal may range from 8:1 to 3:1, and is preferably from 6:1 to 4:1. The adsorbents are useful both in stationary and mobile applications for the removal of CO2 from the exhaust gas stream and the recovery of substantially pure CO2 that can be compressed for temporary storage pending its ultimate disposition.
Following desorption of the CO2, the regenerated adsorbent can then be revised, without substantial loss in adsorbent capacity during a significant number of cycles.
[34] As noted, the magnesium compound is preferably MgO, and the at least one salt of a Group IA metal is preferably a carbonate, and/or a nitrate salt.
An especially preferred composition of the invention is MgO:Na2CO3:NaNO3, where the molar ratio of Mg:Na is about 4.8. Salts of Li, K, or Rb may replace the sodium salts in the preferred composition.
[35] The mixed salt sorbents of the invention can be made via, e.g., a gelation reaction, as in Example 1, or preferably a precipitation reaction. To elaborate, a magnesium salt and a Group IA metal salt are prepared in solution form, and combined to form a reactive mixture. This reaction may optionally be carried out with a precipitating agent. The salts are chosen such that, upon reacting with each other, MgO or MgCO3 is formed in the precipitate. Preferably, a highly soluble Mg compound is used, such as MgO itself, Mg(OH)2, or most preferably, Mg(NO3)2. As noted supra, MgCl2 or Mg(CH3C00)2 may also be used. Once the Mg salt is chosen, the skilled artisan may determine what Na salt or salts will react therewith to produce the desired MgO! MgCO3.
[36] The examples, supra, describe two methods for making the sorbent, i.e., gelation and co-precipitation, or "precipitation." In the latter method, an Mg salt, and a Group IA metal salt, are dissolved in water, and then a precipitant is added, resulting in precipitation of the sorbent powder. This is the easier method to use in the lab, but it requires large quantities of water, batch yields are low, and material preparation is

7 expensive. The gelation methodology requires greater care, with the "trade off' of less water usage, higher yields, and reduced preparation costs.
[37] Following preparation of the sorbent powder, it can be made into an extrudate, either via adding a binder, such as boehmite, or via special preparative techniques known in the art, which result in a loss in sorbency; however, the technique is useful for keeping pressure drops low, in packed beds, and for rendering handling of the material easier.
[38] In tests summarized here, higher capacity was found in extrudates without binder, and these achieved high CO2 loads (about 20 wt% at 300 C). The crush strength of such binder-free extrudates without binder was found to be 0.51 MPa, equivalent to those extrudates prepared with boehmite (0.55 M_Pa).
[39] On an industrial level, extrusion runs are nearly continuous, with continuous extraction and conveyance on, e.g., a belt convection dryer. Again, in experiments only summarized here, different drying rates were tested, and it was found that a rate of about 0.3 C/minute results in an extrudate with reasonable crunch strength.
Increasing the drying rates did in fact decrease the strength of the resulting product.
[40] The reaction is carried out with concentrations of the reactive salts which provide for a ratio of Mg:Group IA metal of from 3:1 to 8:1, most preferably from 4:1 to 6:1. The choice of ratios is one left to the artisan because, as noted supra, by varying the ratio one produces sorbents with different properties. Knowing the conditions under which the sorbent will operate will determine the ratios employed. Optionally, a precipitating agent may be added to facilitate the reaction, such as NaNO3.
The precipitating agent is preferably a salt of a Group IA metal.
1411 The invention also comprehends methods for removing CO2 from a gas or gas mixture, such as an exhaust gas stream produced by the combustion of a hydro-carbon fuel, by contacting the gas or gas mixture with the mixed salt sorbent described supra, at a temperature which ranges from about 100 C to about 450 C, preferably from about 250 C to about 350 C, for a time sufficient for the sorbent to remove a portion of

8 the CO2 therefrom. In practice, the sorbent will become "saturated" by the CO2, and this can be deteimined by measuring the content of CO2 in gas after it has contacted the sorbent and comparing this value to the amount of CO2 in the gas prior to the contact.
When it is evident that CO2 is not being removed to the extent desired, the sorbent can be regenerated by indirect heat exchange, e.g., with the hot exhaust gas stream which is at an elevated temperature, e.g., about 500 C. Again, by measuring the amount of CO2 which = is contained in the exiting gas, the ordinary skilled artisan can determine when the sorbent has been regenerated and can be reused.
[42] In practice, the removed CO2 can be compressed and temporarily stored on board the mobile source until it is removed for permanent disposition, such as underground storage. Alternatively, the CO2 can be put to use in various ways.
For example, the process described herein, in addition to removing CO2, results in the accumulation of condensed H20. These two species can be fed into a reactor or other means for generating fuel which, in turn, can be used, e.g., to power the CO2 producing system described herein. Of course, if the fuel is not used as it is produced, it, also can be accumulated and stored for later use.
[43] The stored CO2 can be used, e.g., as a refrigerant gas, and then channeled for cooling and/or air conditioning. The captured CO2 gas released from the adsorbent can be reacted with the condensed water recovered from the exhaust gas stream to form a hydrogen-based fuel which can in turn be used, e.g., to power the ICE of the vehicle in which the CO2 and H20 were produced. Any solar energy to which the vehicle or other mobile source was exposed can also be stored and used to facilitate this reaction.
[44] In addition, means can be provided in the system for cooling the CO2, thereby permitting its use as a coolant or refrigerant for use in the system.
[45] Other facets of the invention will be clear to the skilled artisan and need not be reiterated here.
[46] The terms and expression which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and

9 expression of excluding any equivalents of the features shown and described or portions thereof, it being recognized that various modifications are possible within the scope of the invention.

Claims (24)

WE CLAIM:

1. A method for removing CO2 from a gas or gas mixture produced by combustion of hydrocarbon fuel by an internal combustion engine on a mobile source comprising contacting a mixed salt composition which comprises: (i) MgCO3 or MgO and (ii) at least one salt of a Group IA
element, wherein (i) and (ii) are present in a molar ratio of from about 8:1 to about 3:1, with said gas or gas mixture produced by said internal combustion engine at a temperature of from about 100°C to about 450°C for a time sufficient for said mixed salt composition to remove a portion of CO2 therefrom and releasing adsorbed CO2 to regenerate said composition by heat exchange with hot exhaust gas discharged from said internal combustion engine.

2. A method for removing CO2 from a gas or gas mixture produced by combustion of hydrocarbon fuel by an internal combustion engine on a mobile source, comprising contacting a mixed salt composition which comprises (i) a magnesium salt and (ii) at least one carbonate or nitrate of a Group IA element, wherein (i) and (ii) are present in a molar ratio of from about 8:1 to about 3:1, with said gas or gas mixture produced by said internal combustion engine at a temperature of from about 100°C to about 450°C for a time sufficient for said mixed salt composition to remove a portion of CO2 therefrom and releasing adsorbed CO2 to regenerate said composition by heat exchange with hot exhaust gas discharged from said internal combustion engine.

3. The method of claim 1, wherein (i) and (ii) are present in a ratio of from 6:1 to 4:1.

4. The method of claim 1, wherein (i) is MgO.

5. The method of claim 1, wherein (ii) is a carbonate or nitrate.

6. The method of claim 1, wherein (ii) is a Na, Li, K or Rb salt.

7. The method of claim 1, wherein (ii) is a sodium salt.

8. The method of claim 1, wherein said mixed salt composition is MgO:Na2CO3:NaNO3.

9. The method of claim 1, wherein said temperature is from about 250°C to about 350°C.

10. The method of claim 1, further comprising contacting said mixed salt composition with an inert gas to remove any CO2 adsorbed therein.

11. The method of claim 10, wherein said inert gas is N2.

12. The method of claim 10, comprising contacting said inert gas to said mixed salt composition at a temperature of about 500°C.

13. The method of claim 1, wherein said mobile source is a transportation vehicle.

14. The method of claim 13, wherein said transportation vehicle is an automobile, truck, bus, train, or boat, and the hydrocarbon fuel is gasoline or diesel fuel.

15. The method of claim 2, wherein (i) and (ii) are present in a ratio of from 6:1 to 4:1.

16. The method of claim 2, wherein (i) is MgO.

17. The method of claim 2 wherein (ii) is a Na, Li, K, or Rb salt.

18. The method of claim 2, wherein said salt composition further comprises a sodium salt.

19. The method of claim 2, wherein said temperature is from about 250°C
to about 350°C.

20. The method of claim 2, further comprising contacting said mixed salt composition with an inert gas to removed any CO2 adsorbed therein.

21. The method of claim 20, wherein said gas is N2.

22. The method of claim 21, comprising contacting said inert gas to said mixed salt composition at a temperature of about 500°C.

23. The method of claim 2, wherein said mobile source is a transportation vehicle.

24. The method of claim 23, wherein said transportation vehicle is an automobile, truck, bus, train, or boat, and the hydrocarbon fuel is gasoline or diesel fuel.